A basic requirement for a space-based antenna is the precise alignment of the radiating elements and/or the reflector (the antenna figure). The alignment required is usually 0.1 of the wavelength or better. For an X-band system, this tolerance is 3 mm. There have been a number of alignment techniques proposed; and in general, a real time sensing system is to be preferred because of its adaptability to a servo system for an actively controlled correction of the antenna figure. A number of existing systems have the disadvantage of relying on fiber optic components which embrittle in space due to low temperatures and radiation.
In a deployable membrane antenna of the present assignee, a figure monitoring system was devised using three photogrammetric cameras that simultaneously photograph points on the flat membrane antenna. Then the photographic film is developed and the imagery analyzed photogrammetrically to determine the motion of the points. The procedure produces the required accuracy of 0.1 wavelength, but is limited by the frame rate of the camera. Further, the process is not done in real time.
The problem of alignment of large ground-based radio telescopes has been with us for many years. The ground-based alignments are facilitated by the presence of terra firma on which to locate instrumentation and easily accessible associated computer equipment. Of course, this is not possible in space.